1. Field of the Invention
The present invention relates to a differential gear and a correcting method thereof used for a vehicle.
2. Description of the Prior Art
A prior art differential gear is disclosed, for instance, in the Japanese Patent Publication No. 57-4536. According to the differential gear of the publication, when a differential function is not necessitated, for instance, when a wheel of a vehicle is caught in mud and another wheel has no driving force, causing the vehicle to hardly escape from the mud, a friction clutch as a differential limiting means disposed on a differential case and on one of side gears of the differential gear is pushed and coupled by hydraulic pressure not to perform differential function and transfer the same rotational speed to the right and left wheels.
However, such a prior art differential gear particularly requires, for operating the differential limiting means, an oil tank, an oil pump, and a hydraulic system such as piping for connecting them together. These elements occupy a large space, complicating the layout of other components, and, for a four-wheel drive vehicle, inevitably increasing the height of the vehicle, causing a problem when the vehicle is driven on a rough road.
Although it is possible to introduce engine oil into the differential limiting means to eliminate need for an exclusively dedicated oil tank and oil pump, elongated piping which is indispensable for this purpose still leaves the problem of spacing. In addition, the longer the piping, there is a more the possiblility of a malfunction, with a result deteriorated reliability.
Conventionally, a differential gear is arranged in a power transmission system for a vehicle to allow, at the time of turning, an outer wheel to rotate faster than an inner wheel to achieve the rotational difference between the left and right wheels, thereby securing a smooth turn.
The differential gear is disadvantageous in driving the vehicle under some conditions. For instance, if a wheel is caught by mud, torque cannot be transmitted to another wheel due to the constitution of the differential gear to disable the vehicle from escaping from the mud.
To avoid such a disadvantage, there is a differential limiting mechanism such as the one disclosed in the Japanese Patent Publication No. 57-4536 which manually limits the differential rotation between two driving axles when the differential rotation exceeds a predetermined value.
The differential limiting mechanism adjusts the differential rotation caused between both the axles by optionally adjusting the coupling force of clutch disks. A clutch operating mechanism for adjusting the coupling force is accommodated within a differential carrier.
According to such a differential gear, the clutch operating mechanism for controlling the differential limiting mechanism is assembled integrally with the differential carrier so that the differential carrier shall be of an exclusively type for the purpose. Therefore, it is designed separately from a general-type differential carrier, losing compatibility thereof, increasing the cost, and complicating the management of parts.
Further, such a prior art mechanism necessitates hydraulic pressure to move pistons for pushing the clutch disks.
Since the power transmission unit described in the above uses hydraulic pressure for the differential control, the design of the differential carrier, etc., for supporting the power transmission unit is changed greatly. In addition, the differential limiting operation is executed by a driver through a pedal pushing operation so that the amount of operation may depend on a sense of the driver, destabilizing correctness in the operation.
In addition, the prior art mechanism described in the above utilizes hydraulic power that complicates its structure by its hydraulic piping and seals.
To cope with such a problem, there has been proposed a simple structure which realizes the differential limiting operation by driving an arm by a motor. However, when the arm is driven directly by the motor, the motor is subjected to reaction from the differential limiting mechanism through the arm so that the durability of the motor may be deteriorated.
If the arm is driven by the motor through a spring, the reaction may be absorbed by the spring, improving the durability of the motor. However, when the spring is interposed between the motor and the arm, the fatigue of the spring may change the necessary control amount of the motor, destabilizing a correct control.